[R-sig-Geo] Isohyet maps where the inverse distance method does not work correctly

[Stefano Sofia]

Dear list users,
I implemented a code for creation of snowfall isohyets through the inverse distance method applied to a Digital Elevation Model.
Within a big area (approximatively 2.000 km2) I use only six stations, I would expect the snowfall esteem getting smaller with higher distances from the stations.
In my case the snowfall values remain constant all over the area.

This is the relevant part of my code (df_prec is the data frame that contains the coordinates of the stations and the snowfall cumulates).
Could somebody be so patient to spot where is my (big) mistake? (Here attached - as exception - the file picture1.pdf is an example of the result.)

Thank you for your attention and your help
Stefano Sofia



## LOADING SHAPEFILE FOR AREA5
Shape_area <- readOGR(dsn="area.shp", layer="area")
Shape_area_projection <- "+init=epsg:32633 +proj=utm +zone=33 +datum=WGS84 +units=m +no_defs +ellps=WGS84 +towgs84=0,0,0"
Shape_area <- spTransform(Shape_area, CRS(Shape_area_projection))

## LOADING DEM
area_DEM raster("dem.tif")
area_DEM <- projectRaster(area_DEM, crs = CRS("+init=epsg:32633"))

## EXTRACTING THE ELEVATION VALUES TO MY POINTS
df_prec$ExtractedElevationValues <- extract(x=area_DEM, y=df_prec)

## CREATING A NEW GRID
newgrid <- as(area_DEM, "SpatialGridDataFrame")
newgrid <- raster(newgrid)
names(newgrid) <- "ExtractedElevationValues"

## Inverse Distance Weighted
if (sum(df_prec$Cumulata) > 0)
{
  OK_rain <- gstat(formula=snowfall_cumulate~ExtractedElevationValues, data=df_prec, locations=newgrid)
  rain_rast <- interpolate(newgrid, OK_rain, xyOnly=FALSE, na.rm=FALSE)

} else
{
  rain_rast <- raster(df_prec)*0
}




         (oo)
--oOO--( )--OOo----------------
Stefano Sofia PhD
Area Meteorologica e  Area nivologica - Centro Funzionale
Servizio Protezione Civile - Regione Marche
Via del Colle Ameno 5
60126 Torrette di Ancona, Ancona
Uff: 071 806 7743
E-mail: stefano.sofia using regione.marche.it
---Oo---------oO----------------

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